Bacillus thuringiensis var israelensis (Bti) produces four insecticidal toxins used around the world to control disease-borne and harmful dipterans populations: Cry4Aa, Cry4Ba, Cry11Aa and Cyt1Aa. They each present their lethal effect on different dipterans, but combined, they generate a synergistic activity and a reduced resistance is observed. Though these synergies are well documented and supported by toxicity bioassays, little is known regarding the cellular and molecular mechanisms of these synergies. Here, by using freshly isolated midguts from the mosquito Aedes aegypti, an important malaria vector, and glass microelectrodes, we measured the electrical potential of the apical membrane when exposed to these toxins alone or in combination. We observed a depolarisation when treated with Cyt1Aa and Cry4Aa. Toxin mixture assays only revealed a faster depolarisation of the membrane when the above two toxins were combined together, and a variety of responses with other toxin mixtures. Microspectrofluometry using the calcium probe Fura-2 on an immortal cell line from Anopheles gambiae (Ag55) showed massive effect of Cyt1Aa, but very little effect of the Cry toxins alone or in mixture. Microspectrofluometry experiments were also conducted on freshly dissociated cells from Aedes aegypti. Though these experiments are innovative and the results preliminary, it was observed that some cells responded differently to Cyt1Aa and Cry4Ba, showing the various ways these toxins affect cells, by inducing either intracellular calcium change, or by entirely losing the probe, or by cell lysis. The mixture of these toxins is very efficient and almost instantaneous.